Two-stroke engines (i.e., two-cycle engines), which are also used as the engines of sea-going vessels, are reciprocating engines in which the strokes of compression (including air intake) and expansion (including combustion and air discharge) are completed in the strokes of a single reciprocation of a piston inside a cylinder. For example, in a diesel-type gas engine in which diesel fuel oil and fuel gas are supplied concurrently via high-pressure injection, the fuel gas and fuel oil are injected in the vicinity of top dead center in the latter half of a compression stroke, and this fuel gas is then ignited so that the fuel gas is reliably combusted inside the cylinder.
As an example of this type of gas engine, technology has been disclosed (see, for example, Patent document 1) in which high-pressure (for example, 250 bar (25 MPa)) fuel gas is injected from a fuel gas valve that is fitted onto a cylinder cover positioned in the vicinity of top dead center.
In a gas engine that uses this type of high-pressure injection, the air inside the cylinder is compressed while the scavenging port and the exhaust valve are in a closed-off state, and combustion is then induced by injecting fuel gas directly into this high-temperature, highly compressed air. Accordingly, in a gas engine that employs high-pressure injection, because it is necessary to supply fuel gas when the pressure inside the combustion chamber in the latter half of the compression stroke is high, the fuel gas must be supplied at a higher pressure than that inside the combustion chamber, so that a high-output pressure booster is required. Moreover, in addition to initial costs such as this, increases in operating costs and problems about safety have also occurred. Therefore, a low-pressure injection two-stroke engine has been proposed (for example Patent documents 2 and 3) that, while the pressure inside the cylinder is still comparatively low during the initial stages of the compression stroke, injects fuel gas directly into air that has been taken in via the scavenging port without increasing the pressure of the fuel gas.